In an isotonic contraction, tension remains unchanged and the muscle's length changes. Isotonic contractions differ from isokinetic contractions in that in isokinetic contractions the muscle speed remains constant. While superficially identical, as the muscle's force changes via the length-tension relationship during a contraction, an isotonic contraction will keep force constant while velocity changes, but an isokinetic contraction will keep velocity constant while force changes. A near isotonic contraction is known as Auxotonic contraction.

There are two types of isotonic contractions: (1) concentric and (2) eccentric. In a concentric contraction, the muscle tension rises to meet the resistance, then remains the same as the muscle shortens. In eccentric, the muscle lengthens due to the resistance being greater than the force the muscle is producing.

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This type is typical of most exercise. The external force on the muscle is less than the force the muscle is generating - a shortening contraction. The effect is not visible during the classic biceps curl, which is in fact auxotonic because the resistance (torque due to the weight being lifted) does not remain the same through the exercise. Tension is highest at a parallel to the floor level, and eases off above and below this point. Therefore tension changes as well as muscle length.

There are two main features to note regarding eccentric contractions. First, the absolute tensions achieved can be very high relative to the muscle's maximum tetanic tension generating capacity (you can set down a much heavier object than you can lift[citation needed]). Second, the absolute tension is relatively independent of lengthening velocity. This suggests that skeletal muscles are very resistant to lengthening, thereby allowing very high levels of tension to develop as can occur in isometric exercise.

Muscle injury and soreness are selectively associated with eccentric contraction (Fridén et al. 1984; Evans et al. 1985; Fridén and Lieber, 1992). However, muscle strengthening may be greatest using exercises that involve eccentric contractions because high levels of tension generate the signals for muscle strengthening.

This is almost an isotonic contraction because there is some fluctuation towards the end of the contraction. For example, the heart's ventricles contract to expel blood into the pulmonary artery and aorta. As the blood flows out, the previous built-up load is decreased and hence less force is required to expel the rest of the blood. Thus the tension is reduced.